Fix crash disabling monitor with locked surface

dwl.c

@@ -736,6 +736,8 @@ cleanupmon(struct wl_listener *listener, void *data)
 	wl_list_remove(&m->frame.link);
 	wl_list_remove(&m->link);
 	wl_list_remove(&m->request_state.link);
+	if (m->lock_surface)
+		destroylocksurface(&m->destroy_lock_surface, NULL);
 	m->wlr_output->data = NULL;
 	wlr_output_layout_remove(output_layout, m->wlr_output);
 	wlr_scene_output_destroy(m->scene_output);